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New 'Smart' Glass Passes Light, Blocks Heat - 2004-08-12

Where there is light, there is heat. At least that is the way things usually occur in nature. But British scientists have separated these two phenomena, at least when it comes to glass. They have developed a glass coating that passes light, but blocks heat. The innovation could have a beneficial impact on energy use.

If you look out a window on a sunny, cold day, the glass radiates heat that adds to your comfort. But if you stand in front of that window on a sunny, hot day, the radiated heat is unwelcome, unless you have air conditioning to overcome it.

Scientists at University College London have formulated a glass coating that knows the difference in the outside temperature and limits the amount of heat a windowpane radiates inside. You could call such treated glass seasonally savvy. Its co-inventor calls it intelligent glass. Researcher Troy Manning says it can cut down on indoor air conditioning bills.

"This, obviously, has effects on the cost of maintaining a building, but also in the broader sense, reducing global warming from using less electricity and such," he said.

Conventional tinted glass blocks heat, but also blocks light. You might be cooler, but the room is also darker, which might force you to turn lights on and use extra energy.

The glass coating developed by Mr. Manning and colleague Ivan Parkin allows visible light of all wavelengths through at all times, but when outdoor temperatures rise above room temperature, or 29 degrees Celsius, the coating begins to reflect away invisible infrared wavelengths, the source of the heat.

The coating is a derivative of the compound vanadium oxide, which undergoes changes in its molecular structure to switch its reflectivity. Normally, a pure vanadium oxide coating does this at a searing 70 degrees Celsius.

"Now, obviously that's a little bit hot. You're going to start to cook by the time it starts to reflect away the infrared radiation," Mr. Manning said.

Mr. Manning and his colleague solved this problem. They report in the Journal of Materials Chemistry that they lowered the switching temperature of the vanadium oxide coating by adding the metal, tungsten, in a concentration that had eluded other scientists.

"You can adjust the amount of tungsten to your particular climate just by adding a little bit more, or a little bit less tungsten. You can adjust what temperature this occurs at," Mr. Manning said.

To limit the cost of manufacturing the coated glass, the two researchers devised a new method of depositing the film. It occurs at standard air pressure, while the glass is being made. Other thin film deposition processes occur after the glass is cooled, and require expensive vacuum chambers.

Currently, the tint is an unattractive yellow green, but the University College London scientists are investigating techniques to suppress the color, perhaps by adding another coating that will neutralize the hue.

"I think within maybe five years, you could start seeing intelligent windows in your office or in your home," Mr. Manning said.